The present invention relates to dense, ceramic-metal composites that approach theoretical density and methods for producing them. More particularly, the invention relates to ceramic-metal composites that are formed of chemically incompatible components that also may exhibit non-wetting behavior.
Ceramic materials are combined with metals to form composite compositions that exhibit exceptional hardness and toughness yet are often light in weight in comparison with metals. Achieving the best potential characteristics for any ceramic-metal composite requires that the composite produced is substantially void-free and capable of achieving theoretical density for a given starting mixture. Also, since a key advantage of such ceramic-metal composites is hardness, it is desirable to maximize the ceramic component content Preferably, at least 50 percent by volume of the composite composition is ceramic which composition has been difficult to fully densify heretofor. The metal component lends toughness to the ceramic-metal composite and is additionally a key element in obtaining void-free densification. It is also desirable that the finished densified compact is substantially similar chemically and in ceramic grain size to the starting mixture. Such similarity is important to achieving composites that have predictable and uniform characteristics.
Obtaining fully densified ceramic-metal composites for such mixtures has not been achieved in the past because of the relatively difficult nature of combinations of the ceramics and metals of interest. Many of the metals and ceramics are non-wetting and thus difficult to fully densify by processes that require metal flow under influence of capillary forces into the voids between ceramic particles. Also, many of the ceramics and metals are "incompatible", in the sense that they react with one another during conventional densification processing that utilizes higher temperatures as an aid to overcoming non-wettability difficulties. As a result of such reactivity the finished composite may include new components or phases, the presence of which generally adversely affects the character of the composite product.
The prior art discloses densification of ceramic-metal composites by means of a number of techniques that include hot pressing, hot isostatic pressing (HIP) and explosive compaction. Thus, Schwarzkopf in U.S. Pat. No. 2,148,040 discloses a hot pressing process for densifying a ceramic-metal mixture involving heating the mixture to a presintering temperature that is defined as 10-15 percent below the melting temperature of the entire mixture. The resulting spongy, porous structure, preferably still hot, is then extruded through an orifice at 71.1-213.3 thousand pounds per square inch (kpsi) pressure (489.0-1469.6 MPa). The pressure increase causes the lower melting point metal component to start to flow thus filling the interstices between the ceramic particles.
A difficulty with the composites produced by Schwarzkopf was that they were not reliably fully dense or uniform because of inadequate temperature control. Also, the extrusion pressing step severly limited the types of composites that could be produced to simple shapes.
A complexly shaped ceramic-metal composite is produced by making a homogeneous slurry mixture of the component powders which slurry is then, for example, cast upon a mold of desired complexity and dewatered to form a green body or compact. The compact is heated to a high temperature to produce a final densified compact but substantially below 100 percent of theoretical density. It was quickly recognized that the application of high pressure would aid in further compaction. It was also recognized that conventionally available pressures of up to a few thousand psi were inadequate to achieve full density for a number of ceramic-metal compacts.
Interest, therefore, turned to explosive compaction processes in which pressures could be applied to ceramic-metal composites on the order of many thousands of pounds per square inch applied in milliseconds. Thus, McKenna et al. in U.S. Pat. No. 2,648,125 surrounds a ceramic-metal compact with a body of liquid and subjects the body of liquid to explosive pressure that isostatically applies 50-60 kpsi pressure to the compact. McKenna notes that it is desirable that the pressure not be developed too rapidly and that maximum pressure is best achieved within 25 to 50 milliseconds. Brite et al, in U.S. Pat. No. 3,276,867 discloses a process for densifying a mixture of powdered uranium oxides or nitrides, etc. and a powdered metal such as tungsten, nickel, iron or the like. The process requires heating the mixture to a temperature that is below any reaction temperature between the powders followed by a high energy, high rate compaction, exerting pressures of 250-400 kpsi over 2-6 milliseconds. Zernow et al. in U.S. Pat. No. 3,157,498 employs an explosive technique in which the compact is subjected to short-time high compression which induces a very large adiabatic temperature increase that may be on the order of several thousand degrees K in the compact.
The explosive compaction processes were unsatisfactory for a number of reasons. Process temperatures utilized were difficult to control, often, as in Zernovw et al, resulting in such large increases that adverse phase formation occurred. Composites so processed were generally limited to small sizes and the extreme pressures often caused composite cracking. The industry therefore turned to elevated temperature pressing at somewhat lower pressures, as a means of attaining more uniformily formed composites.
Lichti et al. in U.S. Pat. No. 4,539,175 describes compacting powder material such as a ceramic-metal body by heating the body to 926.degree. C.-2204.degree. C. and isostatically pressing at 20-120 kpsi.
In forming void-free metal parts from metal powders Nyce in U.S. Pat. No. 4,591,482 initially heats a metal compact to a temperature 10-20 percent lower than sintering temperature. A pressure of 1-2 kpsi is applied to densify the compact and is said to cause a temperature spike that forms small amounts of liquid in the compact that assists in collapsing remaining voids to achieve a substantially fully dense finished part. The temperature spike is described as bringing the compact back to the sinter temperature but only for 5-10 minutes in order to avoid significant grain growth which leads to weakening of the product.
These relatively lower pressure processes tend to employ relatively high temperatures that, in combination with duration of the process, produce multiple phases in incompatible, that is, reactive ceramic and metal systems. As noted earlier, the presence of these phases can be detrimental to finished products qualities.
Recent work focuses more directly upon the mechanisms thought to be involved in the compaction process. Thus, Halverson et al. in U.S. Pat. No. 4,605,440 teaches that in many ceramic-metal systems, densification is improved where a composite is subjected to sufficient temperature such that a liquid metal phase is formed that has a low contact angle of the liquid phase on the solid ceramic phase. This condition is termed wetting and satisfies the capillarity thermodynamic criterion for the system. Halverson describes fully dense boron carbide aluminum composites that are prepared by sintering at a temperature of 1180.degree. C.-1200.degree. C. where wetting of the ceramic component via the aluminum metal component occurs. However, the products produced by Halverson include a number of ceramic phases that differ from the starting materials, including AlB.sub.2, Al.sub.4 BC, AlB.sub.12 C.sub.2, AlB.sub.12 and Al.sub.4 C.sub.3, that adversely affect the mechanical properties of the composite product. These undesirable ceramic phases develop because of the incompatibility between boron carbide and aluminum at the sintering temperature and appear because the reaction rates of aluminum with B.sub.4 C are higher than the rate of the densification process.
Pyzik et al. in U.S. Pat. No. 4,702,770 focuses upon the reactiveness or "incompatibility" characteristics of many ceramic-metal systems at elevated temperatures, particularly those temperatures related to achieving wettability. Pyzik produces composites that consist chiefly of boron carbide, aluminum and minor amounts of other ceramic phases, generally avoiding the multiphase results of Halverson. In Pyzik's process, the kinetics of the chemical reaction between B.sub.4 C and Al are reduced by sintering the B.sub.4 C ceramic component at above 2100.degree. C. For example, a porous green body of the B.sub.4 C is formed, sintered at 2100.degree. C. and then infiltrated with aluminum at a temperature above 1150.degree. C. The method permits some control over the rate of reaction, but does not avoid formation of all undesirable ceramic phases. Additionally, if the metal used is an alloy, the high temperature required for infiltration typically completely changes the composition of the metal found in the composite, e.g., an aluminum alloy of Al, Zn, Mg would change composition at an infiltration temperature of greater than 900.degree.-1000.degree. C. through evaporative losses of Zn and Mg.
In summary, the technologies of densification of ceramic-metal composites by pressing techniques, particularly for chemically incompatible and non-wetting ceramic-metal systems, fail to reliably produce fully densified composites. Predictability of product characteristics is low where the pressing techniques involve higher temperatures. The failure in the art is due to a lack of understanding of how the results achieved in the densification process are influenced by interaction between the wettability characteristics and the incompatibility characteristics of the ceramic and metal components sought to be densified. The more recent work of Halverson et al. teaches the necessity for achieving wetting of ceramic by metal by employing high temperature processing. However, the results achieved at these high temperatures due to chemical reaction between the incompatible components generally causes fast depletion of the metal and often formation of undesirable new phases. The Pyzik et al. process achieves wetting while reducing formation of multiple ceramic phases but requires separate processing steps at high temperatures for the ceramic phase.